Pre-heat the oven to 350º F, and line two jelly roll pans (16x10x1) with parchment paper.
In a large bowl, mix together the butter and sugar.
Add the egg and vanilla to the butter mixture, mixing well.
In a separate bowl, whisk together the flour, baking powder, cocoa powder, and salt.
Alternate adding the dry ingredients and the milk into the butter mixture, beginning and ending with the dry ingredients.
Divide the cookie dough in half and spread each half into the prepared jelly roll pans. The dough will be thin but should cover the bottom of each pan completely.
Bake the cookie dough at 350º F for about 10-12 minutes, switching racks halfway through. Then allow the sheets to cool for about 20 minutes.